The present invention relates generally to finishing of the surfaces of workpieces, and more particularly is directed to the use of finishing media loaded into a barrel with the workpieces.
Finishing the surface of a workpiece usually involves polishing and abrading. Abrasion refers to the removal of larger portions of the surface, primarily to alter the overall contour of the surface. Abrasion is often performed in a wet process for a grinding, deburring, aggressive smoothing or other material removal operation. Polishing refers to the removal of small portions of the surface of a workpiece, in a scratchlike manner, primarily to alter the visible finish. Polishing is often performed in a dry process, resulting in surfaces with reflectivity approaching the quality obtained from manual buffing. Usually, automatic polishing is accompanied by at least a small amount of abrasion due to the manner in which it is performed.
Methods for automatically finishing the surface of workpieces employ a tub into which workpieces and finishing media are loaded. The tub is moved to impart motion to its contents, and the resulting contacts between the media and workpieces remove portions of the surfaces of the workpieces. Automatic finishing methods include rotary barrel finishing, vibratory barrel finishing, centrifugal barrel finishing and centrifugal disk finishing.
Rotary barrel finishing relies on gravitational forces. During rotation, the contents of the barrel move upwards until gravity causes the contents to slide downwards. The majority of the finishing occurs when the contents slide down. The contacts between the media and workpieces tend to be long scratches similar to those obtained using a buffing wheel. This technique is good for smoothing sharp exterior edges and corners (radiusing), but is not particularly effective for inside surfaces.
Vibratory barrel finishing relies on kinetic energy. A vibratory motion is imparted to the contents of the barrel. The finishing occurs during the short strokes of contact between the media and workpieces. This technique is reasonably good for polishing interior surfaces but is not particularly effective for corner or edge finishing. Also, vibratory finishing does not produce a particularly refined finish.
Centrifugal barrel finishing relies on centrifugal pressure. The barrel is rotated while it revolves around an axis, exposing the contents of the barrel to high centrifugal forces. The finishing occurs when the media press on the workpieces. This technique is good for producing refined surfaces in short times. This technique is also appropriate when the identity of each workpiece must be maintained, as each workpiece may be loaded into one of several barrels which simultaneously rotate around their respective axis and revolve around a central axis.
Centrifugal disk finishing also relies on centrifugal pressure. Here, a containment vessel has a rotating disk as a base and a non-rotating cylindrical vertical wall. Media and workpieces are thrown against the wall and slide down. Finishing occurs both when the media press on the workpieces while they are pressed outward and during the downwards sliding. This technique is good for precision finishing in short times, but requires a large amount of monitoring.
Traditional finishing media include hardwood or resin preforms used with abrasive paste, and plastic or ceramic shapes with embedded abrasive. Substantial deterioration of the media occurs during finishing due to the abrasive action of the media upon itself, such as between two preforms or two media shapes. Typically, plastic finishing media lose their mass at a rate of about 3% per hour of use, and ceramic finishing media lose their mass at a rate of about 3-5% per hour of use. Thus, such media are not durable.
When the preforms or shapes impact the surface of a workpiece, a portion of the surface of the workpiece may be abraded or removed from the workpiece. Sometimes this is desirable, as when removing marks or radiusing. However, in some cases, a workpiece has been carefully brought to its present size and shape, and it is desirable only to polish the workpiece, that is, not to abrade or cut down its surface. With conventional media, if the finishing process is controlled so that media contacts do not abrade the workpiece surface, then the finishing intervals become very long, rendering the finishing process relatively expensive.
If the workpiece has an intricately contoured shape, its interior surfaces may not be adequately polished. For example, if the surface includes a U-shaped region, conventional media tends to abrade the tops of the U-shape, but not reach the surface of the bowl at the base of the U-shape.
The preforms or shapes may have tips. When a tip perpendicularly contacts a workpiece surface, the tip digs a pit in the surface of the workpiece. The finished surface has a scratch pattern of peaks and valleys which diffuse or diffract light, resulting in a dull, foggy, matte finish, quite unlike a bright, shiny, highly reflective finish that is often desired.
FIG. 1 shows a spherical workpiece 100 being finished by conventional media 105A, 105B, 105C. Media 105B is seen to be sliding along the surface of workpiece 100, creating a long scratch 110, which desirably finishes the surface of workpiece 100.
The workpiece 100 has a U-shaped socket 130. Media 105C is seen to be eroding the edges of the socket 130. It will be appreciated that even if one of the tips of media 105C entered into the socket, negligible finishing occurs, as it is not possible for the media to slide along the surface of socket 130.
A tip of media 105A contacts workpiece 100 normal to the surface thereof and digs a small pit 120. In FIG. 1, pit 120 is enlarged for ease of illustration. During finishing, the surface of workpiece 100 becomes undesirably pitted.
A portion 150 of the surface of workpiece 100 is shown enlarged. The surface contains pits 151 and long scratches 152, corresponding to the action of media 105A and 105B, respectively.
It is expected that certain abrasives will break down during a finishing interval, so that the finishing interval begins with coarse abrading and concludes with finer polishing. However, this type of finishing cannot be precisely controlled. Furthermore, this type of finishing is not linear with time, that is, during a six day polish interval, the finishing during an hour of the first day is substantially different than during an hour of the sixth day.
Workpieces may be sensitive to the size of abrasive used in a finishing process. Specifically, a certain range of abrasive size may cause skin fractures perpendicular to the surface of a workpiece, giving the workpiece an undesirable shattered look. During the remainder of the finishing interval, the workpiece surface must be abraded sufficiently to remove these fractures, lengthening the finishing interval and changing the size of the workpiece. Alternatively, the finishing process must be controlled so as to remove abrasives in the undesired size range.
Multiple step finishing processes which do not substantially rely on abrasive breakdown have been used for attaining smooth surface finishing of metallic articles or parts. Typically, a first step, abrasive cutdown, removes excess material and provides a coarse finish, while a second step, burnishing, provides a smooth finish and a third step, polishing, provides a finely polished surface. Sometimes a fourth step, waxing, is used to produce a surface with maximum reflectivity.
U.S. Pat. No. 2,185,262 (Lupo) describes a process for finishing metallic articles in a tumbling barrel including a first step of tumbling the articles with hard bony pellets, such as vegetable ivory chips, bone chips, synthetic resin chip or hard tree root chips, and a hard coarse abrasive, such as ground pumice, emery or carborundum of 180 to 200 mesh, to effect a cutting operation for removing tool, grinding or sand marks. In a second step, the articles are tumbled with hard bony pellets and a hard fine abrasive, such are pumice, emery or carborundum of 320 to 400 mesh to effect polishing, and, in a third step, the articles are tumbled with fibrous fragments, such as wood pegs, including a fine abrasive of 500 to 800 mesh to impart high luster to the metal articles.
The process described in Lupo has several drawbacks. The pellet and wood peg media are abraded during finishing. The workpieces are abraded during each step, that is, more surface portions are removed than minimally necessary for polishing. The fibrous fragments, namely, the wood pegs, are rigid enough to dig pits in the surface of the workpieces which may be, e.g., malleable metals. Complex surfaces are not uniformly polished. A finishing process takes a long time, since a rotary barrel is used.
U.S. Pat. No. 3,504,124 (Kittredge et al.) relates to a finishing process carried out in water in a vibratory barrel, using media having a hardness which depends on temperature. Articles to be finished and the media, comprising a rigid plastic binder with abrasives having average particle diameters below 15 microns such as alumina, quartz or silicon carbide, are loaded into vibratory equipment for a first finishing operation at low temperatures of about 35.degree. to 50.degree. F. The temperature of the water is increased to about 100.degree. to 125.degree. F. in a second finishing operation, which produces articles having a finish in the range of one to 5 microinches (0.025 to 0.13 microns). A third step of final polishing is indicated as necessary, but no particular way of performing this final polishing is provided.
The process described in Kittredge et al. has several drawbacks. Importantly, a final polishing step, such as manual polishing, is required. A water supply is necessary, including a way to control the water temperature in a range from very cold to warm. The finishing media are not durable. The workpieces are abraded during each step. Complex surfaces are not uniformly polished.
At present, there is no known method of finishing surfaces which can be accomplished in a short finishing interval, uses durable media, polishes workpieces with minimal abrasion, polishes complex surfaces, provides a lustrous and highly reflective surface, is easy to control and is linear with time.